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M2. Inducing Electron Transitions.
M1. Controlling Electron TransferAnalyze electron transfer
between coupled systems.
Explore the effect of electron transitions in solid systems.
The central goal of this unit is to apply and extend central concepts and ideas discussed in this course
to design chemical systems to harness energy.
Unit 8How do use chemical systems to
harness energy?
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Module 1: Inducing Electron Transitions
Central goal:
To explore the effect of electron
transitions in solid chemical systems.
Unit 8How do we use chemical
systems to harness energy?
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The Challenge
In many chemical systems, electron transitions between different energy levels lead to the
transformation of energy into different forms (heat, light, electrical current).
TransformationHow do I change it?
How can we control these types of
transformations?
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Electronic Levels
Electron transitions between different energy levels may be induced by providing energy to a
chemical system.
In isolated atoms and molecules, the energy states in which electrons exist are
clearly quantized.
E
Transitions between levels only occur when the appropriate E is absorbed or released.
E
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As atoms combine into larger molecules, the
energy difference between the available electron energy levels
decreases.
Energy Bands
E
# of interacting atoms
1 2 3 4 20
In solids, with ~1023 atoms, the energy
difference becomes negligible, and
continuous “energy bands”
are formed.
E
Valence band(Lowermost filled)
Conduction band(Uppermost empty)
Energy Gap (Eg)
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IConductivity
Electrical conductivity depends on the existence of empty energy levels that e- can access:
E
Metal
The energy cost for e- to jump from the VB to the CB is
negligible.
VB
CB
Semiconductor
The Eg can be overcome by
thermal vibrations or UV-vis-IR light.
Eg ~ 60-300 kJ/mol
Insulator
Eg > 300
kJ/mol
Very large Eg.
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ISemiconductors
The metalloids Si and Ge are semiconductors at room temperature, and they form the basis for computer
processors and other electronic devices.
Other “composite” semiconductor materials have been developed by mixing different chemical
elements. However, these composites tend to have an average number of valence electrons equal to 4,
as Si and Ge.
Let’s Think
Which of these composite materials are likely to be semiconductors?
GaAs CdS InP GaSe
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Band Gap
The energy gap Eg between valence and conduction
bands is a critical feature of a given semiconductor.
Semiconductor
The Eg can be overcome by
thermal vibrations or UV-vis-IR light.
Eg ~ 60-300 kJ/mol
E
VB
CB
The Eg depends on the types and relatives
amounts of the different atoms that compose the
system.
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What periodic trends do you detect for the band gap of semiconductors? Hint: Analyze
families of compounds with one common element.
~atomic size
Let′s Think!
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Periodic Trends
~atomic size
Eg increases as the interaction between atoms becomes either more covalent:
Smaller size More electron
density overlap larger Eg
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Periodic Trends
~atomic size
Eg increases as the interaction between atoms becomes more
ionic:
Larger More ionic
character larger Eg
Al = 1.5
Ga = 1.6
Mg = 1.2
Cd = 1.7
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IDoping
Adding very small amounts of impurities (ppm) to an intrinsic semiconductor can increase its
conductivity by a factor of a million.
E
Instrinsic
Si, Ge
VB
CB
Carriers (e-)
E
n-type
Si + P (impurity)
VB
CB Adding atoms with
5 valence e- introduces e-
in donor levels that are
close to the conduction
band.
Donor level
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Doping
Conductivity can also be increased using atoms with fewer valence e- than the host.
E
Instrinsic
Si, Ge
VB
CB
Carriers (h+)
E
p-type
Si + Al (impurity)
VB
CB Adding atoms with
3 valence e- introduces
empty levels that e- can
occupy close to the valence
band.
Acceptor level
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p-n JunctionsE
n-type
VB
p-type
CBMobile e- in a n-type
semiconductor are in higher potential energy states than mobile e- in p-type systems.
What happens if we put them in contact (p-n junction)?
e- flow from the n to the p side until equilibrium is reached (the Efield at
the interface stops the flow).
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Imagine now that the p-n junction is connected to a battery as shown:
a) What would you expect to happen? Will e- move? If yes, in which direction?
b) What would happen if we reverse the connections? Will e- move? If yes, in which direction?
Let′s Think!
hole
e-
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Diodes
Reverse biasNo current flows
Forward biasCurrent flows
E
VB
CB
Energy in the form of light may be emitted as e- fall to lower E levels.
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LED/Photocells
In a Light Emitting Diodes (LED), electrons emit light in
the UV-vis-IR region when they transfer from the CB to the VB in moving across the junction.
In a photocell, light photons are absorbed by electrons in the VB and transferred to the CB. This creates an electric field that can be used to generate a current.
E
VB
CB
n-typep-type
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Solar Cells
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I Assess what you know
Let′s apply!
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ILet′s apply!
An LED is made with a combination of different
materials.
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ILet′s apply!
Design a cheap full LED device that emits:Red (620-750 nm) Green (495-570 nm)
or Blue (450-495 nm) light.
Material Eg (J)
Ge 1.06 x 10-19
Si 1.79 x 10-19
GaAs 2.28 x 10-19
AlGaAs 3.06 x 10-19
GaP 3.62 x 10-19
SiC 4.23 x 10-19
E = h= hc/h = 6.626 x 10-34J-s c = 3.00 x 108 m/s
a) What semiconductor would you use?
b) How would you dope it?
c) What other materials would you use?
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Let′s apply!
Polyepoxide
LeadSiC Blue
GaP Green
AlGaAs Red
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Explain something that you learned in this module to other
person in the class.
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Semiconducting systems can be used to transform light energy into electrical energy, and vice versa, by
inducing e- transitions between energy bands.
Exploring Electronic Structure
Summary
E
Valence band(Lowermost filled)
Conduction band(Uppermost empty)
Energy Gap (Eg)
The energy gap Eg can be controlled by
changing the composition of the
semiconductor
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Summary
Doping and Junctions
Semiconductors are normally “doped” with other substances to change their electric properties.
Junctions formed with p- and n- types are elementary "building blocks" of almost
all semiconductor electronic devices such as diodes, transistors, solar
cells, and LEDs.
E
n-type
VB
p-type
CB
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Are You Ready?
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Electronics
A company interested in producing semiconductors for diverse electronic devices wants to know what binary material to produce to generate a semiconductor with the smallest
band gap given the available resources.
Elements Available Binary material with an
average # valence e- = 4, involving the largest
atoms:
InSb